1. Field of the Invention
The present invention relates to noise rejection systems, and more particularly to a system and method for rejecting repetitive noise from an information signal.
2. Problem to be Solved
Communication systems are often subject to repetitive background noise. For example, automobile muffler systems, manufacturing machinery, vehicle, boat and airplane engines and other source of repetitive noise can interfere with acoustic signal receiving devices located near the noise source such as microphones, telephones, hydrophones, vibration sensors, or electronic transducers. In particular, microphones in emergency vehicle communication systems associated with police cars fire trucks and ambulances receive not only a user""s voice, but also a repetitive background noise generated by the emergency vehicle siren. Such repetitive background noise often overpowers a user""s voice so that a user""s message is difficult to understand. If voice-activated communication systems are employed, background noise increases the difficulty in recognizing voice commands that automatically activate and deactivate the communication system.
Microphone noise rejection systems have been developed to minimize the level of background noise relative to the level of the desired information or voice signal. Such noise rejection systems typically comprise dual microphones in which a first microphone primarily receives background noise and a second microphone primarily receives both background noise and an information or voice signal. The noise signal is then added to or subtracted from the information signal in order to cancel noise from the information signal. For example, U.S. Pat. No. 5,381,473 discloses a noise-rejection system that uses two microphones to receive respectively a noise signal and a source signal. The noise and source signals are supplied to a differential amplifier to cancel noise from the source signal. Similarly, U.S. Pat. No. 6,072,881 also discloses a system in which a first microphone primarily receives background noise and a second microphone primarily receives both background noise and an information or voice signal.
The use of a separate microphone for each distinct noise source, as described in the patents mentioned above, substantially increases the per-unit cost of such noise rejection systems. Furthermore, installation of such systems is difficult, time consuming and costly since a separate microphone must be strategically located in proximity to each distinct noise source. As a result, an inordinate amount of cables or wire and connectors must be used to ensure that noise signals emanating from all distinct noise sources are eliminated.
It is therefore an object of the present invention to provide a noise rejection system that substantially eliminates repetitive background noise from an information or voice signal without the disadvantages inherent in the prior approaches to noise rejection.
In one aspect, the present invention is directed to a method of rejecting noise introduced into an information signal, comprising the steps of receiving an information signal having an information component and a repetitive noise component having a varying characteristic frequency, delaying the information signal by a reference delay time that is equal to a predetermined number of periods of the characteristic frequency to form a delayed information signal, subtracting the delayed information signal from the information signal to form a processed information signal in which the information component is substantial and the noise component is negligible, determining the amplitude of the noise component of the processed information signal, and varying the delay of the information signal as the characteristic frequency of the noise component changes in order to maintain the amplitude of the noise component of the processed information signal at a predetermined level.
One advantage of the present invention is that the adaptive noise rejection system does not require separate microphones or other acoustic signal detection device for each distinct noise source.
Another advantage of the present invention is that the noise rejection system does not suffer from the phase alignment problems inherent in adding or subtracting information and noise signals in order to remove the noise component from the information signal.
Other objects and advantages of the present invention will become apparent in view of the following detailed description and accompanying drawings.